(CNN) -- CNN projects Mitt Romney will win all three of Tuesday's Republican presidential primaries, including Wisconsin, where a loss for Rick Santorum may damage his ability to convince Republicans that he can stop Romney from getting the nomination.

Romney's wins in Wisconsin, Maryland and the District of Columbia will put him past the halfway mark to the 1,144 delegates needed to clinch the nomination and add to a wide delegate lead that he holds over the other major GOP presidential candidates, according to CNN estimates.

"Thank you to Wisconsin, Maryland and Washington, D.C. We won them all," Romney told supporters Tuesday night in Milwaukee. "This has really been quite a night. We won a great victory tonight in our campaign to restore the promise of America."

The former Massachusetts governor is expected to take the majority of Wisconsin's 42 delegates and Maryland's 37 delegates, with some designated solely for the winner and the rest to be awarded proportionally. He will take all 16 delegates at stake in the District of Columbia.

GOP delegate count | Delegate calculator

With 15% of the vote reported in Wisconsin, Romney had 41% and Santorum, a former U.S. senator from Pennsylvania, had 39%. U.S. Rep. Ron Paul of Texas had 11%, and former House Speaker Newt Gingrich had 6%. Exit polls showed Romney with a wider margin in Wisconsin: 43% support to Santorum's 35%.

In Maryland, with 19% of the vote reported, Romney had 49%. Santorum had 30%, Gingrich had 11% and Paul had 9%.In the District of Columbia, with 11% of the vote reported, Romney had 68%. Paul had 15%, Gingrich had 12%, and Santorum was not on the D.C. ballot.

Maryland results | D.C. results

Pre-primary polls appeared to show Wisconsin's primary was the only one of the three where Santorum had a chance to win. And analysts said Wisconsin might be Santorum's final chance to slow Romney's march toward the GOP nomination.

"In a race where none of the 'must-wins' have actually turned out to be 'must-wins,' Wisconsin looks to be as close as Republicans have been to functionally wrapping up the nominating contest," GOP strategist Gentry Collins, a former political director for the Republican National Committee and the Republican Governors Association, said before the primaries.

Exit polls in Wisconsin, where 42 delegates are up for grabs, showed familiar patterns for the top two Republican candidates.

Wisconsin exit poll | Maryland exit poll

As in previous contests, Wisconsin's early exit polls show Romney doing better among higher-earners and Santorum better with lower-income voters. Among exit-poll takers making $100,000 to $200,000 annually, Romney led 55%-30%, with Santorum winning the under-$30,000 voters, 44%-34%.
The Wisconsin exit polls show Santorum is more popular in rural areas and Romney in urban areas in Wisconsin. According to the exit polls, Santorum is winning the rural vote, 37%-27%, and Romney is winning big-city voters, 43%-23%.

About 24% of the vote in Wisconsin has come from urban areas and 22% from rural areas, according to the exit polls. The rest is suburban, where the primary could be decided.

Wisconsin's primary is open to any registered voter min the state, regardless of party. Exit polls show that 58% described themselves as Republicans, 30% said they were independent and 12% said they were Democrats.

Santorum was doing best among self-described Democrats with 32% of their vote, while Romney had 22% and Paul had 16%.

Romney had 571 delegates before Tuesday's primaries, according to CNN's estimate. That's more than twice the 264 delegates Santorum held. Gingrich and Paul trailed well back.

President Barack Obama, meanwhile, clinched the Democratic presidential nomination Tuesday with primary wins in Maryland and the District of Columbia, CNN projected. Unlike the Republicans, Obama faces no serious opposition in his race.

According to CNN's delegate estimate, the president had 2,735 of the 2,778 delegates needed to secure his party's nod before Tuesday's contest. He is expected to win most or all of the 119 delegates at stake in Maryland and D.C., as well as the 100 delegates at stake in Wisconsin, where the polls close at 9 p.m. ET.

On the Republican side, since Romney's double-digit victory in Illinois two weeks ago -- followed by a wave of some of the Republican Party's major names and elder statesmen endorsing him and urging a quick conclusion to the divisive nomination battle -- the conversation seems to have changed: the GOP front-runner increasingly being called the inevitable nominee.

Romney and Obama seemed to be turning their attentions to November's general election, with Romney blasting the president during a campaign stop in Wisconsin and Obama mentioning Romney in a speech for the first time this year.Speaking at a restaurant in Waukesha, Wisconsin, Romney suggested Obama wants to duck responsibility "for what's happened in this country," saying the president should get full credit or blame for "what's happened in this economy, and what's happened to gasoline prices under his watch."

"It is time to have somebody who will take responsibility, and if I am president, I will not only get things right again, I will take full responsibility for my errors and make sure that people understand we have a president in the White House again where the buck will stop at his desk," Romney said, while standing with House Budget Committee Chairman Paul Ryan, a prominent Wisconsinite who endorsed Romney.

State-by-state primary/caucus results

Later Tuesday, Obama mentioned Romney while slamming a House-passed budget proposal that Ryan drew up and Romney embraced.

Obama, speaking at a media luncheon in Washington, said the plan, which would lower tax rates and cut spending while reforming the Medicare and Medicaid government-run health care programs, was "thinly veiled Social Darwinism" and "antithetical to our entire history as a land of opportunity and upward mobility for everyone who's willing to work for it."

"One of my political opponents, Gov. Romney, has said that he hoped a similar version of this plan from last year would be introduced on day one of his presidency," Obama said. "He said that he's very supportive of this new budget and he even called it marvelous, which is a word you don't often hear when it comes to describing a budget."

Romney responded, in an interview with Milwaukee radio station WTMJ, that Obama is engaging "in the most incendiary rhetoric he can find out of an attempt to ignore the fact that we can't keep on spending a trillion dollars more than we take in every year."

While Romney has focused on Obama, Santorum seems to be downplaying expectations, saying in an interview Monday on "Piers Morgan Tonight" that he knew that "April would be a very tough month."

But regardless of the increasingly daunting odds facing him, Santorum doesn't sound like a candidate about to end his White House bid.

While Romney is far ahead of Santorum, Gingrich and Paul in the hunt for delegates, exit polls indicate Romney still has a problem locking in the conservative base of the party.

That continues to fuel Santorum's campaign, in which he continually depicts himself as the lone true conservative going up against the Republican establishment and liberal media bias.

"The reason I'm here is because what the establishment is trying to shove down the throats of the folks of this country on the Republican ticket isn't being swallowed. And the reason it's not being swallowed is because they want someone who reflects their values, someone that they can trust, someone who's an authentic conservative," Santorum told voters Sunday in Milwaukee.

Romney still hasn't appeared to leave anything to chance in Wisconsin.

He campaigned in the state over the three days leading up to Tuesday's primary, and his campaign, combined with an independent pro-Romney super PAC, will outspend Santorum and his super PAC by a nearly 4-to-1 ratio to run TV ads in Wisconsin.

That led Santorum to ask Sunday in Mishicot, Wisconsin: "Why is he (Romney) spending $4 million in Wisconsin, if this race is over? If it's over and there's no chance, then why is he bothering even campaigning anymore, if it's over?"

Santorum has some help on the ground in the Badger State.

"Santorum's very capable deputy campaign manager, Jill Latham, ran the Republican Party's turnout programs in Wisconsin through the 2006 cycle and knows the state well. She will be a big tactical asset in Wisconsin, and should be able to plus-up Santorum's chances on the ground," said Collins, who ran Romney's 2008 operation in Iowa but is not taking sides in this cycle.

A Santorum win in Wisconsin on Tuesday could revive the headlines that Romney is having trouble locking up the nomination. But a Santorum loss will most likely increase the chorus of voices for him to drop out.

After Tuesday, there are three weeks before five more states vote on April 24.

Romney appears to be the favorite in four of those states: New York, Connecticut, Rhode Island and Delaware.

The fifth, Pennsylvania, is Santorum's home state, but polls show his once-large lead there has disappeared.

"Do you want to lose your home state? I don't think so. I think if Santorum wants any kind of future, how do you stay in and lose Pennsylvania?" asked Crowley, anchor of CNN's "State of the Union."

Collins agreed, adding, "If it looks like Romney has the nomination locked up before then, Santorum will need to be careful to avoid a serious embarrassment in his own home state."

Santorum and his campaign say there's no talk of dropping out until Romney clinches the nomination. Crowley says any public mention right now of possibly calling it quits would be detrimental to Santorum.

"The minute you show any leg on getting out, your supporters are going to vote for Romney. So you're kind of stuck," Crowley said. "Your words have to be bravado. I think his words on Wednesday will tell us more about his strategy than his words on Monday."

Santorum points toward May, which looks friendlier for him. He could be the favorite in primaries in North Carolina and West Virginia on May 8, Nebraska on May 15, Arkansas and Kentucky on May 22 and Texas a week later.

"May is rich with delegates and strong states for us. By the end of May, we expect this race to be very close to even," Santorum told Morgan.

But it could be too little, too late by May. Sen. Hillary Clinton won most of the final contests against Sen. Barack Obama in the marathon 2008 Democratic presidential nomination battle. But in the end, it didn't matter.

Said Crowley: "Santorum will win some states, but at the moment it's not about winning states. It's about winning the nomination, and that's where it gets difficult for Santorum."

I read through Santorum's column. I already knew his stances on all of those, but having them all compiled together made me realize just how fucking batshit insane he is (not to say that's a recent revelation to me)